In order to relieve the shortage of copper subscriber lines from a telephone central office to a telephone subscriber residence, and also to reduce the length of such lines, access concentrators are frequently employed. Access concentrators employing a digital multiplex carrier to carry many individual voice conversations on a single, or several, multiplex carriers are known as Digital Loop Carriers (DLCs).
FIG. 1 illustrates a block diagram of typical DLC represented generally by the numeral 10. A number of individual twisted pair subscriber lines 12 are terminated by line cards 14. In the case of typical residential telephone service, the lines 12 are terminated by plain-old telephone service (POTS) line cards 14, which provides a number of functions required to operate a telephone or other terminal 16 connected to the line 12 at the subscriber's residence. These functions usually include, but are not limited to:                Battery—supplying power to the subscriber terminal;        Overvoltage—protecting the line card against environmentally caused overvoltages;        Ringing—supplying a ringing signal;        Supervision—detecting when a subscriber lifts the telephone receiver;        Coding—converting the analog line signal into digital representation;        Hybrid—separating the received from the transmitted signals; and        Termination—terminating the line with a required standard electrical impedance.        
These functions are often collectively referred to as BORSCHT.
Each line card produces a signal in a format suitable for being multiplexed with other line card signals onto a common Time Division Multiplexed (TDM) bus 18 although other buses may be employed. Typically, each line signal is sampled at an 8 kHz rate and each sample is converted into a digital codeword, each usually consisting of 8 bits using one of several standard coding formats known as μ-law Pulse Coded Modulation (PCM) or A-law PCM. The resulting bit rate per terminated line is, therefore, 64 kb/s.
The codewords from one terminated line 12 or line card 14 are interleaved with those of other lines 12 and line cards 14 onto the TDM bus 18. The codewords are then commonly transmitted over a backplane to a common element, herein called a Transmit and Receive Unit/Line Interface Unit (TRU/LIU) 20 by assigning one or several timeslots to each line card. The TRU/LIU 20 performs two main functions:                1. Electrically driving and receiving signals to/from the TDM bus 18, clock generation, timeslot control, and the like; and        2. Interfacing to a digital multiplex carrier 21, by means of a Digital Carrier Interface (DCI) 27, which possibly includes but is not limited to line coding, line quality monitoring, clock recovery, synchronization, loopbacks, framing, alarm detection, multiplexing of signaling bits, maintenance channel termination, and maintenance and remote monitoring functions. Common digital multiplex line interfaces include electrical (T1, E1) and optical (OC-3, OC-12) interfaces.        
In order to allow for the electrical testing of subscriber lines 12 and line cards 14, a Test Access Unit (TAU) 30 is typically provided. The TAU 30 is connected to all line cards 14 in the DLC 10 by means of a test bus 32. Test bus implementations and characteristics vary widely depending on the particular embodiment and design of the DLC 10. Typically, in a DLC 10 designed and intended primarily for multiplexing voice services, the test bus 32 characteristics are intended to be electrically suitable for voice signals, but not necessarily for high speed digital signals, such as digital logic or DSL signals.
The TAU 30 is typically controlled by a control unit 40, which may communicate with the TAU 30 and other units in the DLC 10 by means of a control bus 41.
FIG. 2 illustrates the TAU 30 and line card 14 in greater detail. Here a plurality of line cards 14 and the TAU 30 are interconnected for line and circuit testing purposes by the test bus 32.
The test bus 32 typically groups a number of independent conductors together into one or more logical buses. Without loss of generality, the test bus 32 in FIG. 2 has been shown to be logically grouped into four subsets herein labeled, with no intended loss of generality, TestOut2, TestIn2, TestOut4, and TestIn4. This illustrates two relevant concepts:                1. Some line cards communicate with a subscriber terminal using four or more wires and may consequently require 4-wire test buses; and        2. The capability to simultaneously test the subscriber line (known as the test out function) and the Line Interface Circuit (LIC) 11 of the line card 14 (known as the test in function) may be provided by means of distinct subsets of the test bus 32.        
FIG. 2 also shows the constituent functional blocks of a 2-wire line card 14. The TDM and Control Interface (TCI) 13 contains circuitry to electrically interface to the TDM bus 18 and the control bus 41, as well as provide general control functions for the line card 14. The LIC 11 provides the BORSCHT functions described above. The test relay 15 provides a means for electrically isolating the LIC 11 from the subscriber line 12, connecting the LIC 11 to the TestIn2 bus, and connecting the subscriber line 12 to the TestOut2 bus.
Operation of 4-wire line cards (not shown) is a logical extension of the above with respect to test relay connectivity with corresponding 4-wire test buses.
FIG. 2 also shows constituent functional blocks of the test access unit 30. A terminations block 34 contains a plurality of electrical terminations required in order to enable meaningful electrical tests of the LIC 11 or the subscriber line 12 to be performed via the test bus 32. A switch matrix 33 provides appropriate means (typically relays or other electrically controllable switches) to electrically connect one or more subsets of the test bus 32 to one or more of a set of line terminations in the terminations block 34 and/or to the external test line 31 (typically twisted pair), either individually, jointly, or severally. A Control Interface 132 contains circuitry to electrically interface with the control bus 41, provide general control functions, and provide specific control functions to actuate the appropriate relays (or other interconnection devices) in the switch matrix 33. The control interface 132 may also function to relay communications to and from the control unit 40 (see FIG. 1) by means of the control bus 41 to and from an external device connected by means of a control interface port 35. An example of such an external device might be a keyboard and display enabling a craftsperson to control the testing of lines of line cards by means of keyboard commands.
Both the test relay 15 and the switch matrix 33 are responsive to the control unit 40, which in turn may be responsive to commands from a test controller (not shown).
In many instances, it is desirable to upgrade an existing DLC 10 to enable the provision of new services, specifically for Digital Subscriber Line (DSL) services, and especially asymmetric DSL (ADSL). Besides ADSL, there are a variety of DSL services (e.g., symmetric DSL, high rate DSL, very high rate DSL, etc.) that are generally referred to collectively as xDSL. Since the bit rate per subscriber line for such a service is many times that for which a voice DLC was engineered and requires the processing of data protocols, such an upgrade becomes problematic.
In order to provide DSL service to a subscriber already served by a voice-only DLC 10 requires changes to hardware. This change may necessitate the physical reconnection of a subscriber's line to a completely different DSL-capable DLC, if one is available at the site. If a DSL-capable DLC is not available, one may need to be installed to serve that subscriber, if space is available in the remote cabinet. If there is insufficient unused space in the cabinet or there is another reason why a new DLC cannot be installed, the subscriber may be denied DSL service altogether.
It is an object of the present invention to obviate or mitigate some of the above disadvantages.